This invention relates to the preparation of high density zirconium diboride ceramic bodies by the pyrolysis of mixtures comprising zirconium diboride powder and preceramic organosilicon polymers.
Zirconium diboride ceramic bodies are known in the art. They have found particular utility, for example, as wear parts and in the nuclear industry because of their high hardness, resistance to wear and nuclear properties. Early methods for producing these bodies involved hot-pressing zirconium diboride powder at temperatures up to 2300.degree. C. This method, however, has a number of disadvantages. In the first place, the method does not produce green bodies and, as such, does not allow for green machining. Secondly, the process is expensive in that it requires the use of high pressure during sintering. Finally, it is difficult to form bodies of complex size and shape by hot pressing methods.
An alternative approach to producing zirconium diboride bodies is to use fugitive binders to form green zirconium diboride bodies and then pressureless sintering these green bodies. In this approach, however, the binder must be pyrolyzed out of the bodies. As such, the process takes additional time and the ceramic bodies undergo significant shrinkage which may result in warpage or cracks.
Yajima et al. in U.S. Pat. No. 4,289,720 teach a process for the formation of ceramic fired bodies. The process comprises molding mixtures of organosilicon polymers and ceramic powders to form green bodies followed by pyrolyzing the green bodies to form ceramic bodies. The reference, however, lists over 150 ceramic powders (including zirconium diboride) whereas the examples only show densification of silicon carbide, silicon nitride and boron carbide. Moreover, the reference only teaches temperatures up to 2000.degree. C. (temperatures in the range of 1550.degree.-1800.degree. C. are preferred) (col. 9, lines 1-4). The present Applicant has discovered that such temperatures are not sufficient to complete the polymer pyrolysis. As such, the density of the bodies in the reference is less than those of the present application.
The object of the present invention is to provide a method for producing high density, high strength zirconium diboride ceramic bodies. The present inventor has unexpectedly discovered that such ceramics can be obtained by sintering a mixture comprising a preceramic organosilicon polymer and zirconium diboride powder.